Boreholes are frequently drilled into the Earth's formation to recover deposits of hydrocarbons and other desirable materials trapped beneath the Earth's surface. Traditionally, a well is drilled using a drill bit attached to the lower end of what is known in the art as a drillstring. The drillstring is traditionally a long string of sections of drill pipe that are connected together end-to-end through rotary threaded pipe connections. The drillstring is rotated by a drilling rig at the surface thereby rotating the attached drill bit. Drilling fluid, or mud, is typically pumped down through the bore of the drillstring and exits through ports at the drill bit. The drilling fluid acts both to lubricate and cool the drill bit as well as to carry cuttings back to the surface. Typically, drilling mud is pumped from the surface to the drill bit through the bore of the drillstring, and is allowed to return with the cuttings through the annulus formed between the drillstring and the drilled borehole wall. At the surface, the drilling fluid is filtered to remove the cuttings and is often recycled.
In typical drilling operations, a drilling rig and rotary table are used to rotate a drillstring to drill a borehole through the subterranean formations that may contain oil and gas deposits. At the downhole end of the drillstring is a collection of drilling tools and measurement devices commonly known as a Bottom Hole Assembly (BHA). Typically, the BHA includes the drill bit, any directional or formation measurement tools, deviated drilling mechanisms, mud motors, and weight collars that are used in the drilling operation. A measurement while drilling (MWD) or logging while drilling (LWD) collar is often positioned just above the drill bit to take measurements relating to the properties of the formation as the borehole is being drilled. Measurements recorded from MWD and LWD systems may be transmitted to the surface in real-time using a variety of methods known to those skilled in the art. Once received, these measurements will enable those at the surface to make decisions concerning the drilling operation. For the purposes of this application, the term MWD is used to refer either to an MWD (sometimes called a directional) system or an LWD (sometimes called a formation evaluation) system. Those having ordinary skill in the art will realize that there are differences between these two types of systems.
A popular form of drilling is called “directional drilling.” Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction. Directional drilling can be advantageous offshore because it enables several wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which may increase the production rate from the well. A directional drilling system may also be beneficial in situations where a vertical wellbore is desired. Often the drill bit will veer off of a planned drilling trajectory because of the unpredictable nature of the formations being penetrated or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on course.
A traditional method of directional drilling uses a BHA that includes a bent housing and a positive displacement motor (PDM) or mud motor. The bent housing includes an upper section and a lower section formed on the same section of drill pipe, but are separated by a bend in the pipe. Instead of rotating the drillstring from the surface, the drill bit in a bent housing drilling apparatus is pointed in the desired drilling direction, and the drill bit is rotated by a mud motor located in the BHA.
A mud motor converts some of the energy of the mud flowing down through the drill pipe into a rotational motion that drives the drill bit. Thus, by maintaining the bent housing at the same azimuth relative to the borehole, the drill bit will drill in a desired direction. When straight drilling is desired, the entire drill string, including the bent housing, is rotated from the surface. The drill bit may angulate with the bent housing and drills a slightly overbore, but straight, borehole.
Positive displacement motor (PDM) power sections include a metal (typically steel) rotor and a stator. The stator is typically a steel tube with rubber molded in into a multi-lobed, helixed profile in the interior. The stator tube may be cylindrical inside (having a solid rubber insert of varying thickness), or may have a similar multi-lobed, helixed profile machined into the interior so that the molded-in rubber is substantially uniform thickness (i.e. “even wall”). Power sections, whether solid rubber or even-wall, are typically uniform throughout the length of the power section. That is, they are either all-rubber or all-even-wall over the entire length of the multi-lobed profile.
Motor failure during directional drilling can be a significant and undesirable event. One mode of motor failure is rubber chunking. Elastomeric materials in the mud motor provide a seal between the rotor and the stator. Without this seal, the motor does not operate efficiently and may fail altogether. In mud motors, as they currently exist, the elastomer sustains undesirable lateral forces between the rotor and the stator as the rotor turns. It may be desirable to determine a way to reduce or eliminate the excessive lateral forces sustained by the elastomer.
It has been observed that the majority of chunking happens on the down hole part of the rubber lining. The second main chunking occurrence is at the up and down hole portions on the same stator. Potential causes of this chunking have been hypothesized as aggressive differential pressures, motor stalling, junk damage, poor rotor/stator matching, and elastomer quality degradation.
These potential causes do not explain why most of the chunking happens or starts mainly on the bottom portion of the stator. One potential explanation for this pattern is the concentrated presence of side forces on the rubber at the downhole part followed by the uphole part of stator. Potential contributing factors for the concentrated side force at the bottom of the stator during drilling of curve section may include: side forces resulting from the bending of motor section to fit in the directional hole; side force resulting from the combined effect of hydraulic thrust on the rotor and the misalignment between the rotor axis and the transmission shaft; side force resulting from the combined effect of the torque on the rotor and the misalignment between the rotor axis and the transmission shaft; and side force resulting from inertial forces produced by the transmission shaft.